well if you want serious horsepower, youre going to have to get a brand new drivetrain IMO.
Also, id go for the 4A-GZE motor, its very well built, and very strong, the block can take a lot of pressure, and the supercharger with the right pulleys can pull almost 17psi of boost. The supercharger is clutched too, which completely eliminates lag, which to me is what makes a great track car.
Supercharging the normal 4A-GE motor to me isnt a good idea, the "z" is specially designed to take the added pressure of forced induction (Which is what you will need if you want to compete with guys who have much larger, much more modern V-TEC motors.
If youre looking for pure straight line speed, I aggree with the people who have allready posted, see if you can drop in a MKII tubo motor, they have alot of power, but I personally wouldnt do it, as that it will harm the weight distribution, which is what makes MR2s so wonderful. But pound for pound, dollar for dollar, the MKII turbos make more power than the 4A-GZEs
I personally have no idea how much such an engine swap would cost, or how long it woudl take...
If I was you, id put the money into things like harnesses, roll bars, coilovers, and brake rotors/pads. But those arent the things you do for pure speed.